A composite material obtained by coating a polyolefin-based resin on a metal material such as steel to impart water resistance and chemical resistance is being widely used for building materials, home appliances, furniture and the like. In particular, an organic composite metal material having coated on the surface thereof a polyolefin resin such as polyethylene and polypropylene is excellent in corrosion protection performance, and therefore has been applied over a wide range of fields requiring corrosion prevention for a long period of time in a severe environment, such as in seawater or heated water.
However, the polyolefin-based resin does not have a polar functional group in the molecular structure and furthermore has high crystallinity, and therefore there is a problem that the adhesion to a metal surface is very poor. Various methods for solving this problem have been studied. Particularly, in the case of a steel material, for example, a surface treatment technique such as chromate treatment and phosphoric acid treatment, and a technique where a polyolefin resin layer modified with a functional group having high polarity as represented by an acid anhydride group and a carboxyl group used as the adhesive, are well known as a means for enhancing the adhesion strength between a polyolefin-based resin and a steel material. However, these techniques still have a problem such as insufficient adhesion force, cumbersome process and high cost. For example, a polyolefin resin-laminated metal material as represented by heavy-duty steel has the following problems:
(a) when exposed for a long period of time, separation of the polyolefin resin from the end face may occur (edge delamination),
(b) water-resistant adhesion in a water environment is insufficient, or when galvanic protection is applied, separation in the cathode part (coating work damage part) may occur (cathodic delamination), and
(c) when cooling to room temperature from the bonding temperature or use at a high temperature, the polyolefin resin is readily shrunk and separated (shrinkage delamination).
As a result, corrosion of a metal substrate or a steel material sometimes occurs in these separated portions.
In order to solve these problems, there is disclosed a technique where a chromate-treated steel surface is further treated by silane coupling, titanium coupling, an epoxy primer or the like and then a modified polyolefin resin and a polyolefin resin are sequentially laminated and coated thereon (see, Kokai (Japanese Unexamined Patent Publication) Nos. 61-44439 and 62-255141). By this technique, the adhesion force between the adhesive and the steel material is further increased and initial adhesion is greatly improved. However, even by such a countermeasure, when used for a long period of time in a severe environment at high temperature and humidity, the adhesion force decreases and the above-described separations occur in some cases.
In order to solve this problem, there is disclosed a technique where not only by increasing the chemical bonding force between the steel material and the resin sheet, but also by aggressive reduction of the residual stress in the resin sheet is taken into consideration in order to ensure adhesion and thereby prevent the above-described separations (see, Japanese Unexamined Patent Publication (Kokai) No. 2003-12826). More specifically, this is a technique of adding rubber to a polyolefin resin and at the same time, dispersing the rubber in a micaceous form, thereby reducing residual stress. However, in this technique, unless a very strong shearing force is applied to the resin by injection molding or the like, the intended structure cannot be realized, and therefore a sheet is disadvantageously difficult to produce.
There is also a problem that since the polyolefin resin has low surface hardness, the polyolefin resin-coated metal material has poor scratch resistance and abrasion resistance and use thereof is limited in the application, such as building materials, home appliances and furniture. As the means for solving this problem, a technique of coating the polyolefin resin coated surface with a polymethacrylic acid or the like is known (see, Japanese Unexamined Patent Publication (Kokai) No. 2000-15759), but this technique is disadvantageous in that the process is cumbersome or the coat layer separates from the polyolefin resin.
Also, as means for reforming the polyolefin resin, which has many problems as described above when applied to a metal material, a polymer nanocomposite technique of very finely dispersing a layered inorganic material in the polyolefin resin on a nanometer scale is known. This polymer nanocomposite technique is known to enhance the barrier property against corrosion-causing substances, increase the elastic modulus, and reduce the linear expansion coefficient and elevation of thermal deformation temperature (see, Kiyoshi Chujo, Saishin Gijutsu to Oyo of Polymer-kei Nanocomposite (Advanced Technology and Application of Polymer Nano-Composite), page 10, CMC (2001)).
A technique of improving the physical properties of the resin composition by finely dispersing a layered inorganic material in the matrix polymer on a nanometer scale is already publicly known, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 8-333114 and 10-182892. However, this technique has a problem in that the resin composition is significantly embrittled (see, Kiyoshi Chujo, Nanocomposite no Sekai (World of Nanocomposite), page 43, Kogyo Chosakai Publishing, Inc. (2000)) and can hardly be applied to the coating of a metal material of which good workability is an absolute requirement. In order to improve the embrittlement, techniques of adding rubber to the resin composition or finely dispersing an organifying layered clay mineral in the rubber itself on a nanometer scale are known, but these have a problem in that when the dispersion diameter, dispersion structure or the amount of added rubber is improper, the above-described improvement of physical properties is impaired or coating a metal material becomes difficult.
An object of the present invention is to provide a polyolefin-based resin composition for metal coating, which is more excellent in separation resistance, weather resistance and corrosion resistance, and can be industrially produced without difficulty even by an existing apparatus, and is assured of excellent scratch resistance; a resin film using the composition; and a resin-coated metal material.